Optical fibers have been widely used for the propagation of optical signals, especially to provide high speed communications links. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Optical signals carried by optical fibers are processed by a wide variety of optical and optoelectronic devices, including integrated circuits. Optical communications signals in optical fibers are typically in the 1.3 μm and 1.55 μm infrared wavelength bands. Optoelectronic integrated circuits made of silicon are highly desirable since they can be fabricated at low cost in the same foundries used to make VLSI integrated circuits. The optical properties of silicon are well suited for the transmission of optical signals, due to its transparency in the infrared wavelength bands of 1.3 μm and 1.55 μm and its high refractive index. As a result, low loss planar silicon optical waveguides have been successfully built in silicon integrated circuits.
Optical signals traveling in optical fiber frequently need to be coupled to optoelectronic circuits and this can be done through a variety of known techniques and devices. There are many advantages to directly coupling optical signals on fiber with integrated optoelectronic silicon based circuits. The flat end of an optical fiber can be directly connected to the edge of a silicon integrated circuit, so the optical signal can be coupled to a flat end of a planar waveguide. An optical signal in a fiber can be coupled to a planar waveguide through the top surface of an integrated circuit using a grating coupler. The efficiency of such fiber to chip connections depends on many factors, including the number and types of optical modes in the fiber and in the integrated waveguide. Once an optical signal is on a chip, it can be processed either as an optical signal or converted to an electronic signal for further processing.
An optical beam traveling in a single mode fiber (SMF) with circular cross section will typically have two optical modes, with one mode polarized in the x direction and a second mode polarized in the y direction. These two orthogonal polarizations have approximately the same propagation constant and approximately the same group velocity. Some refer to these two modes as a single mode with two polarization components. Within this discussion of the present invention, the two orthogonal polarizations are referred to as two modes.
Similarly, two orthogonal polarization modes are preset in standard forms of polarization maintaining fibers. These two modes have sufficiently different phase and group velocities to prevent light from coupling back and forth between the two modes. However, the differences are slight enough that they can usually be treated in a similar manner to SMF, when used as an input to polarization splitting elements.
In theory and under ideal conditions, there is no exchange of power between the orthogonal polarizations. If an optical signal is directed into only one polarization, then all the power should remain in that polarization. But in actual practice, imperfections or strains in the fiber cause random power transfer between the two polarizations. The total power is thus divided between the two polarizations, and this may not be a problem in some applications, but in many situations, this can be a major problem. In some cases, there can be a great deal of fluctuation and power transfer between the two polarizations. Such random fluctuations could cause the power delivered on one polarization, to be close to zero, which would result in some loss of signal, if only that polarization is being received.
Single mode optical fiber with a circular cross section has two optical modes, although due to the rotational invariance of a single mode fiber, one mode is difficult to describe without referencing the presence of the other. Planar waveguides have a different type of modal configuration, where there are two primary types of modes: the transverse electric (TE) and the transverse magnetic (TM), which describe which field of the mode is oriented purely transversely to the direction of propagation. This is strictly true only for 2 dimensional ideal waveguides, however this naming convention is also used for real world three dimensional waveguides, which are only approximately TE or TM. Future references herein will make the common assumption that quasi-TE or quasi-TM modes are understood as TE or TM modes.
It is difficult to connect an optical signal from an optical fiber to a planar waveguide due to differences in: cross sectional geometry, polarization characteristics and the number of optical modes. An SMF optical fiber has a circular cross section with a core diameter of less than ten microns. A nanophotonic planar waveguide can be substantially smaller, and as a result, contain modes that vary substantially in cross-sectional geometry. An SMF fiber will typically have two polarizations with essentially the same phase and group velocities. The polarizations in a planar waveguide can have very different phase and group velocities, or the planar waveguide could support only a single polarization mode. The number of optical modes in an SMF fiber is two when operated at the appropriate wavelength. A typical planar waveguide can have many optical modes within it, or it could have only one, depending on the design.
Waveguides are designed for use over a particular wavelength range, so a single mode waveguide at one wavelength very often becomes multimodal at substantially shorter wavelengths. When one skilled in the art refers to waveguide operation, it is commonly understood that a particular wavelength range is being referenced with respect to single or multimode operation.
The design process for optical paths comprises construction of maskworks. Maskworks include shapes, layout, data structures, netlists, and alignment marks and other elements of the design which are typically stored as digital data on a computer system. In addition, these electronic representations of the designs are transferred to a set of many physical masks which are used during the fabrication of the components. These many masks are included in the definition of maskworks.
As a result of the many differences in characteristics between optical fibers and planar waveguides, it has been difficult to connect optical signals from one to the other.